 
Avian influenza
The disease
Avian influenza, or bird ‘flu, is a contagious disease of animals caused by viruses that are closely related to human influenza viruses. Avian ‘flu normally infects only birds, but has also infected pigs occasionally and humans rarely.
The H5N1 strain is a highly pathogenic form of avian influenza, which spreads rapidly through poultry flocks, causing serious disease and mortality, often within 48 hours.
The H5N1 virus does not spread easily from birds to humans. Despite the death or destruction of approximately 150 million birds and the huge number of people employed worldwide in the poultry industry, only about 100 people have been infected with bird ‘flu.
H5N1 in poultry poses two main risks to human health:
- The current main risk is direct infection when the virus passes from birds to humans. Although it is difficult to contract from birds, infection with H5N1 virus in humans leads to very severe disease. Of the approximate 100 human cases in the current Asian outbreaks, over half have died.
People are believed to become infected with H5N1 virus by direct contact with infected birds, or surfaces and objects contaminated by bird faeces. There is no evidence that properly cooked poultry or eggs can be a source of infection.
- The second risk is that the H5N1 virus may change to a form that can easily spread from person to person (much like ordinary ‘flu), but retains its severe symptoms. This could lead to a new pandemic.
A new pandemic
For H5N1 to become a pandemic virus it must ‘swap’ or ‘re-assort’ genetic material with a human strain of influenza virus within a person, or pig, infected with both viruses at once. Re-assortment would be identified by a sudden surge of cases with explosive speed.
Alternatively the H5N1 virus could gradually adapt to infecting humans through genetic mutations. Adaptive mutation would be seen as small clusters of human cases with some evidence of human-to-human transmission.
Prevention
To be most effective, a vaccine for influenza needs to closely match the pandemic virus strain. Because re-assortment of human with avian ‘flu viruses can lead to any number of final combinations, no one knows exactly which virus will cause a pandemic. As such, vaccines cannot be prepared in advance.
Treatment
Anti-viral drugs can reduce the severity and length of illness caused by seasonal influenza. If given early, they may also improve the survival prospects of someone infected directly with H5N1.
Antibiotics — Because it is caused by a virus, influenza cannot be treated with antibiotics. However, severe disease is often complicated by secondary bacterial infection of the lungs, and in these cases antibiotics may be used as part of an overall treatment regime.
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